1. Field of the Invention
The present invention is directed to the etching of metal foil for use in electrolytic capacitors and more particularly to a method of controlling the etch pattern on an electrolytic capacitor foil used in implantable cardioverter defibrillators (ICDs).
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Electrolytic capacitors are used in ICDs because they have the most near ideal properties in terms of size and ability to withstand relatively high voltage. Conventionally, an electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. The electrolyte impregnated in the separator functions as the cathode in continuity with the cathode foil, while an oxide layer on the anode foil functions as the dielectric. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, composing a planar, layered, stack structure of electrode materials with separators interposed therebetween.
Since these capacitors must typically store approximately 30-40 joules, their size can be relatively large, and it is difficult to package them in a small implantable device. Currently available ICDs are relatively large devices (over 30 to 40 cubic centimeters (cc)), generally about 12-16 millimeters (mm) thick. A patient who has a device implanted may often be bothered by the presence of the large object in his or her pectoral region. For the comfort of the patient, it is desirable to make smaller ICDs. The size and configuration of the capacitors contribute 9 to 12 cc of the ICD volume.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, increasing the surface area of the aluminum anode foil results in increased capacitance per unit volume of the electrolytic capacitor. By electrolytically etching aluminum foils, an enlargement of a surface area of the foil will occur. As a result of this enlargement of the surface area, electrolytic capacitors, which are manufactured with the etched foils, can obtain a given capacity with a smaller volume than an electrolytic capacitor which utilizes a foil with an unetched surface.
Etching the foil increases the surface area of the foil. A metal foil may be etched according to any method that increases the surface area, such as electrochemical etching, roughing the foil surface mechanically and chemical etching. Electrochemical etching increases the surface area of the foil by electrochemically removing portions of the foil to creates etch tunnels. Electrochemical etching is done according to any known etch process, such as the ones discussed in U.S. Pat. Nos. 4,474,657; 4,518,471; 4,525,249 and 5,715,133 which are incorporated herein by reference.
In a conventional electrolytic etching process, surface area of the foil is increased by removing portions of the aluminum foil to create etch tunnels. The foil used for such etching is typically an etchable aluminum strip of high cubicity. The etch initiation and hence the gain or capacitance of the foil is the result of several variables, such as foil cubicity, thermal oxide on the foil, and the electrochemical reaction. Research indicates that the capacitance can be raised by 2.5 to 3 times if the etch tunnels were arrayed periodically to achieve maximum theoretical surface area. Thus, increased surface area equates to a reduction in capacitor volume from 50-66% which in terms of current ICD volume is a 3 to 4.5 cc reduction.
The ideal etching structure is a pure tunnel-like etching with defined and uniform tunnel diameters and without any undesirable pitting of the foil. As tunnel density (i.e., the number of tunnels per square centimeter) is increased, a corresponding enlargement of the overall surface area will occur. Larger surface area results in higher overall capacitance. However, high gain etching of valve metals for use as anodes in electrolytic capacitors tend to produce very brittle anode foil. Typically the higher the gain of the anode foil, the more brittle the foil. In particular, the brittleness of the foil and its capacitance are both proportional to the depth of the etching and the density of the etch pits, i.e., the number per unit area. Accordingly, the capacitance and thereby the energy density are limited by the brittleness of the formed foil. As the brittleness of the formed foil increases, cracks formed in the foil more easily propagate across the foil, resulting in broken anodes.
Creating a pattern on an aluminum foil surface has been previously demonstrated as a means to successfully increase surface area. For example, U.S. Pat. No. 7,150,767 to Schneider, et al., which is incorporated herein by reference in its entirety, discloses an etching process which applies a holographic image to a photoresist coated on a foil to create a pattern of photoresist on the foil prior to etching. The photoresist pattern on the foil surface allows for positional control of tunnel initiation. Alternatively, U.S. Pat. No. 6,224,738 to Sudduth, et al. and U.S. Pat. No. 6,736,956 to Hemphill et al., which are incorporated herein by reference in their entirety, disclose etching processes which utilize masking to control tunnel initiation. By controlling the position of tunnel initiation, foils are etched more uniformly and have optimum tunnel distributions. This allows for the production of highly etched foils that maintain high strength and have high capacitance. The difficulty arises, however, in attempting to control the pattern on a 1 micron scale. There is a need therefore for a process of etching foil for use in electrolytic capacitors which allows for positional control of tunnel initiation on a micron level.